Configuration options for managing reliable provisional responses, as defined in RFC 3262. disabled: Disables the reliable provisional response services. No reliable provisional responses can be sent or received. supported: Configures calls to use a reliable provisional when required. The first time a provisional response is sent with SDP triggers the use of reliable provisional responses. All provisional before the first one with SDP are unreliable, and all provisional after the first one with SDP are reliable. Note that if the caller forces the use of reliable provisional response through a Require header, all provisional responses will be reliable. If the caller does not support reliable responses, responses are sent normally. required: Requires the use of reliable provisional responses locally and by the peer with which the user is communicating. This forces all such responses sent to be reliable, and forces the peer to support and always send reliable provisional responses.